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This is a place chosen to make available some ideas (on puzzles, patterns, etc)
that seemed unsuitable for posting in a Forum, but some people may find interesting.
For difficult puzzles, possible solutions will not be shown (only hints).
For other puzzles, the solution wil be hidden so that a serious attempt can be made
without direct access to a solution.
(hopefully this will make the puzzles more interesting for newcomers)
(Guidelines for using this site here)
(useful to look up for the meaning of some unfamiliar concept)
I propose the following Classic Sudoku puzzle solvable in two steps.
(found with Sudoku Architect)
After basics
,--------------------------------------------------------------------,
| 1269 129 129 | 268 5 2389 | 4 1378 3789 |
| 7 259 8 | 4 1 239 | 69 35 3569 |
| 1569 3 4 | 68 7 89 | 2 158 589 |
|----------------------+----------------------+----------------------|
| 2589 7 259 | 1 3 256 | 69 4 2689 |
| 1238 128 12 | 9 4 267 | 5 2378 23678 |
| 23459 2459 6 | 257 8 257 | 1 237 2379 |
|----------------------+----------------------+----------------------|
| 18 6 17 | 78 9 4 | 3 25 25 |
| 24 24 3 | 578 6 578 | 78 9 1 |
| 589 589 579 | 3 2 1 | 78 6 4 |
'--------------------------------------------------------------------'
I propose the following tough Classic Sudoku puzzle.
(found with Sudoku Architect)
,-------------------------------------------------------------------------,
9 | 147 257 12457 | 13 6 345 | 8 137 9 |
8 | 19 56 1569 | 7 1359 8 | 2 4 13 |
7 | 1479 3 8 | 2 19 49 | 5 6 17 |
|-------------------------+-----------------------+-----------------------|
6 | 2 9 47 | 5 17 6 | 3 8 147 |
5 | 3478 678 3467 | 139 12379 239 | 1679 1579 12457 |
4 | 5 1 367 | 4 8 239 | 679 79 27 |
|-------------------------+-----------------------+-----------------------|
3 | 139 25 12359 | 6 23459 7 | 149 1359 8 |
2 | 6 4 123579 | 8 2359 2359 | 179 13579 1357 |
1 | 3789 578 3579 | 39 3459 1 | 479 2 6 |
'-------------------------------------------------------------------------'
a b c d e f g h i
Update 1 (June 17, 2026)
The patterns of digits for this puzzle are interesting (in particular: 1, 4, 5)
A good starting move eliminates (4)f9, using almost hidden and almost locked sets.
,-------------------------------------------------------------------------,
9 | 147 257 12457 | 13 6 35-4 | 8 137 9 |
8 | 19 56 1569 | 7 1359 8 | 2 4 13 |
7 | 1479 3 8 | 2 19 49 | 5 6 17 |
|-------------------------+-----------------------+-----------------------|
6 | 2 9 47 | 5 17 6 | 3 8 147 |
5 | 3478 678 3467 | 139 12379 239 | 1679 1579 12457 |
4 | 5 1 367 | 4 8 239 | 679 79 27 |
|-------------------------+-----------------------+-----------------------|
3 | 139 25 12359 | 6 23459 7 | 149 1359 8 |
2 | 6 4 123579 | 8 2359 2359 | 179 13579 1357 |
1 | 3789 578 3579 | 39 3459 1 | 479 2 6 |
'-------------------------------------------------------------------------'
a b c d e f g h i
How to continue ?
Update 2 (June 18, 2026)
,-------------------------------------------------------------------------,
9 | 147 257 12457 | 13 6 5-3 | 8 137 9 |
8 | 19 56 1569 | 7 1359 8 | 2 4 13 |
7 | 179 3 8 | 2 19 4 | 5 6 17 |
|-------------------------+-----------------------+-----------------------|
6 | 2 9 47 | 5 17 6 | 3 8 147 |
5 | 3478 678 3467 | 139 1237 239 | 1679 1579 12457 |
4 | 5 1 367 | 4 8 239 | 679 79 27 |
|-------------------------+-----------------------+-----------------------|
3 | 139 25 12359 | 6 2345 7 | 149 1359 8 |
2 | 6 4 123579 | 8 235 2359 | 179 13579 1357 |
1 | 3789 578 3579 | 39 345 1 | 479 2 6 |
'-------------------------------------------------------------------------'
a b c d e f g h i
After just one placement and basics, a **pretty almost chain** is the most
difficult move for this puzzle, eliminating (3)f9, again with just one
placement and basics.
Update 3 (June 19, 2026)
For move 2: (3=9)f4 - (9=7)h4 - (7=13)d9.h9 eliminates (3)f9, if we can
show that the spoiler (2)f4 also eliminates that target.
After move 2, and its immediate consequences, we get the following
.-----------------------------------------------------------------------.
9 | 147 27 1247 | 13 6 5 | 8 137 9 |
8 | 19 56 56 | 7 139 8 | 2 4 13 |
7 | 179 3 8 | 2 19 4 | 5 6 17 |
|-------------------------+---------------------+-----------------------|
6 | 2 9 47 | 5 17 6 | 3 8 47-1 |
5 | 3478 678 3467 | 139 1237 239 | 1679 1579 12457 |
4 | 5 1 367 | 4 8 239 | 679 79 27 |
|-------------------------+---------------------+-----------------------|
3 | 13 25 12359 | 6 2345 7 | 149 1359 8 |
2 | 6 4 123579 | 8 235 239 | 179 13579 1357 |
1 | 378 578 3579 | 39 345 1 | 479 2 6 |
'-----------------------------------------------------------------------'
a b c d e f g h i
Now, a nice move eliminates (1)i6 [among others], solving the puzzle!
Update 4 (June 20, 2026)
So, the final move comes from the existing guardians of an UR threat.
After finding this solution, it is natutal to ask if such UR could
be applied earlier. Indeed, one can make the same elimination using
the (more crowded) guardians of that UR, but it does not solve the
puzzle in one step [one additional move is needed]. ////
Created: February 12, 2024
Contact: sudo.jco.br@gmail.com